1. Field of the Invention
This invention relates to a soft-start clutch application for equipment configured to be mounted on vehicles or towed by tractors.
2. Background of the Invention
Many types of machines are routinely mounted on vehicle chassis or trailers for easier transportation and use. This is especially true for large agricultural equipment. This machinery includes many types of feed mixers and manure spreaders for use in dairy, beef, and livestock operations. The equipment is typically powered through a transmission PTO (power take-off) driven by the vehicle engine. Depending on the type of transmission available on the vehicle chassis, the way power is supplied to the machine will vary.
In the case of a manual transmission, the gears driving the transmission PTO are manually disengaged while the transmission's clutch is depressed. When the transmission clutch is released, the PTO rotates and the machinery is engaged. Thus, the PTO rotates or stops along with the vehicle's own wheels. In other words, the PTO will only run when the vehicle chassis is in motion, as the PTO is driven by the vehicle transmission gears and not driven directly by the engine. The operation of this type of manual transmission PTO is more difficult than operation of an automatic transmission, so most livestock operations prefer a vehicle chassis with an automatic transmission.
In the case of a traditional automatic transmission, the PTO is powered by the automatic transmission's torque converter. The PTO uses a set of small clutch discs, which allow engagement and disengagement of the PTO. One benefit of using a torque converter to power the PTO is that the torque converter is in effect a hydraulic pump, and therefore inherently allows the PTO to slip somewhat during startup. Because of the slippage produced by the torque converter, the PTO clutch discs experience less of a shock or torque spike upon startup than would occur without the presence of the torque converter. The disadvantage of connecting to the torque converter is that it is continuously slipping, resulting in efficiency losses and less power available at the PTO. In addition, it also means that the PTO operates only when the vehicle is moving or in neutral. Further, the PTO gear ratios available for these types of transmissions are limited, which restricts the machinery which can be mounted on the truck chassis.
In recent years, vehicle manufacturers have incorporated automatic transmissions without torque converters. One example of such a transmission is the new style Allison WORLD CLASS® automatic transmission which drives the PTO gears directly from the engine. This arrangement allows full horsepower to be transferred to the PTO whenever the truck engine is running, providing “live power.” Internal PTO clutches engage and disengage the PTO independently of the vehicle transmission, and no torque converter is present between the PTO and the engine to act as a buffer. Consequently, conventional PTO clutches operating heavy equipment, with running torque loads of 100 hp or above for example, experience severe torque spikes upon startup. Such high torque loads are common for manure spreaders and feed mixers. The torque spike can be in the range of 2-3 times the running torque. Accordingly, such torque spikes cause conventional PTO clutches to wear out faster when operated with an automatic transmission without a torque converter than when operated with a transmission with a torque converter. In other words, the disadvantage of the automatic transmission without a torque converter is that there is no buffer to prevent full engine horsepower from being engaged instantaneously, resulting in considerable shock and stress on both the driven machinery and the PTO itself.
As a solution to the above-noted problems, operators of equipment have resorted to using hydraulic motors to power moving components of equipment. The hydraulic motors allow control of the machinery independently of the rotation or existence of a dedicated PTO. Additionally, hydraulic motor systems allow the driven machinery to rotate in both the clockwise and counter-clockwise directions without special provisions made in gearing. However, hydraulic motor systems are relatively expensive and complex compared to conventional PTO systems.
Further, a diesel truck engine (as is commonly found in agricultural vehicles) typically runs at 1800-2200 RPM, and most machinery is configured to accept a 540 or 1000 RPM input. Thus, the rotational speed of the diesel truck engine must be reduced before connection to the machinery. To achieve this reduction in rotational speed, the transmission PTO is generally specified to transmit as an output a reduced percentage of a given input rotational speed. For example, a 75% reduction PTO would accept an 1800 RPM engine speed and produce a 1350 RPM output to the machinery. However, the gear reduction choices available in conventional PTO's are limited, and generally result in a compromise of the resulting machinery speeds. In addition, the PTO gears used for speed reduction become smaller as the reduction is increased. Therefore, these gears are less capable of carrying the required torque of the machinery.